For example, in a radio communication apparatus installed in a base station for mobile phones, a High Power Amplifier (HPA) is used. For such an amplifier, in order to increase the power efficiency thereof, there is proposed an envelope tracking scheme (also called a power modulation scheme or bias modulation scheme) that modulates a power supply voltage (drain voltage) using an envelope of an RF signal to be inputted (refer to, for example, Donald F. Kimball, et al., “High-Efficiency Envelope-Tracking W-CDMA Base-Station Amplifier Using GaN HFETs”, IEEE Transactions on Microwave Theory and Techniques, Vol. 54, No. 11, November 2006, and Feipeng Wang, et al., “Design of Wide-Band Envelope-Tracking Power Amplifiers for OFDM Applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 53, No. 4, April 2005. In this scheme, since the power supply voltage of the amplifier dynamically changes in accordance with the envelope of the RF signal, when the amplitude of the RF signal is small, the operating power of the amplifier is suppressed. As a result, the power efficiency improves.
In an amplifier of an envelope tracking scheme such as that described above, it is important that an RF signal and a power supply voltage modulated by an envelope of the RF signal reach the amplifier in synchronization with each other and have no timing shift therebetween. The timing shift is mainly caused by a difference in electrical length. Thus, circuit design is performed in consideration of electrical length so as to suppress the shift.
However, in practice, variations occur on a more minute level on a product-by-product basis and a timing shift occurs. Hence, an intended improvement in power efficiency cannot be obtained and distortion occurs in an input-output characteristic of the amplifier. Meanwhile, to eliminate such a timing shift, first, it is considered to measure how much shift there is currently. However, performing such a measurement itself is not easy.